Luminescent phosphor screens are used in cathode ray tubes, for example, television display tubes, electron display devices, imaging devices, for example, image intensifier tubes, etc. Typically, a thin layer of phosphor material containing a luminescence activator is supported on a substrate. The phosphor layer is activated by impingement of an electron beam, and the resulting luminescence is transmitted through the glass substrate to the front of the display.
Phosphor screens, such as those used in image tubes, are made with phosphor powders. The powder is applied to a substrate glass plate or an optical fiber bundle by and one of several known methods, such as settling, brushing, spraying, etc. However, the use of powdered phosphors has numerous disadvantages. The powdered phosphors are not very adherent to the substrate. Typically, some sort of binder must be used to hold the phosphor particles to the substrate, which complicates the production process. Also, the powder is difficult to apply uniformly, leading to low process yields of acceptable units. The powders also have a high surface area which absorbs large amounts of gas from the phosphor constituents. This high outgassing characteristic is extremely undesirable for use in vacuum tubes where pressures of 10.sup.-10 torrs are achieved. Powdered phosphors also have a low optical resolution due to scattering off of the particles.
One proposal has attempted to apply powdered phosphors into intagliated (etched) recesses or wells formed in the core glass of the individual fibers of an optical fiber bundle in order to increase the resolution of the screen. Such an intagliated powdered-phosphor screen is described, for example, in "Intagliated Phosphor Screen Image Tube Project", by Richard J. Hertal, ITT Aerospace/Optical Division, prepared for NASA under Contract NAS5-26417, May 1982. However, this structure has a complicated manufacturing process and all of the other disadvantages of powdered phosphors.
Non-particulate, solid phosphor films have been used in display panel technology. The phosphor layer may be formed as a monocrystalline layer grown on a substrate by liquid phase epitaxy (LPE), or as a thin film deposited by evaporation, sputtering, or vapor deposition (MOCVD/MOVPE) techniques. Such phosphor layers have a relatively high thermal loadability, high resolution, and a low outgassing characteristic in a vacuum tube. However, the solid phosphor films suffer from low optical efficiency, making them undesirable for certain applications, such as image tube screens. The low optical efficiency of solid phosphor films is caused by large internal reflection losses within the film.
U.S. Pat. No. 4,264,408 entitled METHODS FOR APPLYING PHOSPHORS PARTICULARLY ADAPTED FOR INTAGLIATED PHOSPHOR SCREENS issued on Apr. 28, 1981 to J. D. Benham and assigned to The International Telephone and Telegraph Corporation, the assignee herein. This patent depicts a method for applying dry powder particles on an intagliated array of fibers. A thin layer of a heated thermoplastic is formed on the surface of the substrate to uniformly coat the etched pits. The thin layer is then heated to receive phosphor particles which become embedded in the plastic. The plastic layer is removed and the screen is then secured with a binder to fix the particles within the etched pits.
The problem of internal reflection losses in solid phosphor films is illustrated schematically in FIG. 1. An electron beam e.sup.- impinges on the phosphor layer through a metal layer, e.g. aluminum, which is optional in some applications. The electron beam activates an activator element, for example, copper in zinc-sulfide based phosphors, or cerium in yttrium-aluminum-garnet phosphors, which causes electrons to be released and photons from the nearby phosphor material to be emitted with a luminescence effect in all directions. Due to the difference in index of refraction between the phosphor layer and the substrate layer, such as glass, light rays which are incident at an angle greater than the critical angle CA are reflected laterally and become trapped and dissipated within the film. Another form of light loss is attributable to reflections from the substrate layer even within the cone (indicated by the dashed lines) of the critical angle CA, which increases as the light rays approach the critical angle.
As an example, the internal reflection loss due to the refraction difference for ZnS based phosphors grown on Corning type 7056 glass substrate can be in the range of 70% to 80% of the light emitted. Within the acceptance angle, the reflection loss can be another 10% to 25%, for a total loss of about 90% to 95% of the radiated energy. Such high losses result in lower phosphor efficiencies as compared to powdered phosphors. The result is that thin film phosphors have had limited application heretofore.
Some researchers have proposed forming reticulated structures in the phosphor layer to break up the waveguide effect and enhance light output. For example, U.S. Pat. No. 4,298,820 to Bongers et al. discloses the technique of etching V-shaped grooves in square patterns in the surface of the phosphor layer to obtain improved phosphor efficiency by a factor of 1.5. However, the etching process used in Bongers has been found to be impractical for large volume production.
Etching the activated portion of the phosphor layer, with reticulations in the form of trapezoid- or truncated-cone-shaped mesas and overcoating with a reflective aluminum film to form light confining surfaces, has been proposed in the article entitled "Reticulated Single-Crystal Luminescent Screen", by D. T. C. Huo and T. W. Huo, Journal of Electrochemical Society, Vol. 133, No. 7, pp. 1492-97, July 1986, and in "RF Sputtered Luminescent Rare Earth Oxysulfide Films", by Maple and Buchanan, Journal of Vacuum Technology, Vol. 10, No. 5, pg. 619, Sep./Oct. 1973. These trapezoidal mesas improve the light output by a factor of about 2, whereas a factor of 6 or higher would represent output of most of the emitted light. The light output factor could be increased if the mesa size could be made less than 5 microns and the shape made with the optimum reflection angle. However, such a small mesa size requires high lithography resolution and is limited by diffraction from the lithography mask. Crystalline phosphors will also preferentially etch along crystalline planes which are different from the optimum slope angle for the trapezoid shape. Thus, application of trapezoidal mesas and reticulated layers has also been limited.